Lining materials for glass melting furnaces

ABSTRACT

A lining material for glass melting furnaces comprises platinum or platinum alloy as a base material containing osmium as an impurity in an amount no more than 20 ppm. The lining material is used for that part of the melting furnace which comes into contact with molten glass. The low osmium content prevents the formation of bubbles in molten glass, thereby providing high-quality glass products.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a lining material for glassmelting furnaces, which is composed mainly of platinum or platinumalloy, a process for purifying the lining material, a glass meltingfurnace lined with the lining material, and a process for producingglass products.

[0003] 2. Description of the Related Art

[0004] Glass is defective if it contain stones, cords, and bubbles.Stones are crystalline bodies of minerals, and they adversely affect thetransparency and strength of glass. Cords are streaky amorphous bodies,and they adversely affect the refractive index of glass, distorting theimage passing through glass. Bubbles also adversely affect thetransparency of glass. Particularly, bubble wreckage on the surface ofan LCD (liquid crystal display) substrate causes open circuits.Therefore, LCD glass plates and optical lenses should be free of theabove-mentioned defects.

[0005] It is known that stones result from fine powder of refractorymaterial when glass is melted in a melting furnace lined with refractorymaterial, that cords result from refractory material dissolved in glass,and that bubbles result from refractory material in contact with glass.One way to prevent stones and cords is by lining with platinum orplatinum alloy. One way to prevent bubbles is by incorporation of afining agent into raw glass materials.

[0006] The above-mentioned measures are successful to a certain extent.Even a few bubbles are not tolerable for electronic glass (such as LCDglass). Complete elimination of bubbles is impossible even in the caseof lining with industrial platinum of high purity (99.95 wt %), with theremainder (0.05 wt %) being unavoidable impurities. Countermeasuresagainst bubbles are indispensable for further improvement in glassquality.

[0007] Despite their thorough investigation into production facilitiesinvolved, the present inventors were unable to pinpoint the source ofbubbles. They assumed that the major source of bubbles is platinum incontact with molten glass.

[0008] It was found that industrial platinum in actual use varies inresidual impurities depending on its origin and production lot and onwhether or not it contains recycled platinum. An example of analyticaldata is given below. TABLE 1 impurities in industrial platinumImpurities Content Palladium (Pd)  5-13 ppm Rhodium (Rh) 20-40 ppmIridium (Ir)  0-13 ppm Osmium (Os) 30-84 ppm Arsenic (As)  4-14 ppm

[0009] Palladium (Pd) is a stable element and is unlikely to causebubbles. Rhodium (Rh) is an element added to increase the strength ofplatinum and is also unlikely to cause bubbles. Osmium (Os) is liable tooxidation, giving rise to an oxide which vaporizes at a low temperature.In view of this and its high content, osmium is very likely to causebubbles. Iridium (Ir) is oxidized at 800° C. and above (although not soeasily as osmium) and vaporizes at 1000° C. and above. However, in viewof the fact that iridium has long been used as a material for glassmelting crucibles, iridium is unlikely to cause bubbles. Arsenic (As)vaporizes easily or oxidizes easily to form volatile oxides. However, itreadily dissolves in glass and hence is unlikely to cause bubbles.

[0010] Accordingly, the present inventors have experimentally studiedthe relation between the amount of osmium and the number of bubbles. Itturned out that the lower the content of osmium, the less the number ofbubbles. This finding led to the present invention.

SUMMARY OF THE INVENTION

[0011] According to a first aspect of the present invention, there isprovided a lining material for glass melting furnaces, which comprisesplatinum or platinum alloy as a base material containing osmium as animpurity in an amount no more than 20 ppm.

[0012] Limiting the content of osmium as above is effective in reducingthe number of bubbles evolving from the lining material and entering themolten glass. For high-quality glass, the content of osmium shouldpreferably be no more than 10 ppm.

[0013] According to a second aspect of the present invention, there isprovided a glass melting furnace which is lined, at least at a partthereof in contact with molten glass, with a lining material for glassmelting furnaces. The lining material comprises platinum or platinumalloy as a base material containing osmium as an impurity in an amountno more than 20 ppm.

[0014] Lining the glass melting furnaces with the lining materialcontaining osmium as an impurity in an amount no more than 20 ppmreduces the number of bubbles entering the molten glass. Forhigh-quality glass, the content of osmium should preferably be no morethan 10 ppm.

[0015] In one preferred form, the glass melting furnace comprises amelting tank in which glass raw materials are melted, a refining tank inwhich molten glass undergoes a fining treatment, and a forming unit inwhich molten glass is drawn and formed into sheet glass. At least theforming unit is lined with the lining material. The forming unit inwhich molten glass is formed into sheet glass should be free of bubbles.Since the lining material does not evolve bubbles, the forming unitpermits the production of a bubble-free, high-quality sheet glass.

[0016] In another preferred form, the glass melting furnace furthercomprises a rear facility for homogenizing, mixing, and forming defoamedmolten glass. The rear facility may also be lined with the liningmaterial.

[0017] Desirably, the refining tank is also lined at least partiallywith the lining material.

[0018] According to a third aspect of the present invention, there isprovided a process for producing glass products, which comprises thesteps of: refining molten glass in a refining tank; homogenizing themolten glass from the refining tank at a predetermined uniformtemperature in a homogenizing tank; and forming the molten glass fromthe homogenizing tank into a sheet glass in a forming unit. The refiningtank is lined, at least at a wall thereof in contact with the moltenglass, with a lining material for glass melting furnaces, which materialcomprises platinum or platinum alloy as a base material containingosmium as an impurity in an amount no more than 20 ppm, so that the wallevolves no bubbles during refining. The homogenizing tank is lined, atleast at a wall thereof in contact with the molten glass, with a liningmaterial for glass melting furnaces, which material comprises platinumor platinum alloy as a base material containing osmium as an impurity inan amount no more than 20 ppm, so that the wall evolves no bubblesduring homogenization. The forming unit is lined, at least at a wallthereof in contact with the molten glass, with a material for glassmelting furnaces, which material comprises platinum or platinum alloy asa base material containing osmium as an impurity in an amount no morethan 20 ppm, so that the wall evolves no bubbles during forming.

[0019] Bubble-free high-quality glass products can be obtained sincerefining, homogenizing, and forming steps are performed in thefacilities lined with the lining material containing osmium in an amountno more than 20 ppm.

[0020] In one preferred form, the process further comprises the step ofmelting glass in a melting tank prior to the refining step. The meltingtank may desirably be lined, at least at a wall thereof in contact withthe molten glass, with a lining material for glass melting furnaces,which material comprises platinum or platinum alloy as a base materialcontaining osmium as an impurity in an amount no more than 20 ppm, sothat the wall evolves no bubbles during melting.

[0021] In another preferred form, the process further comprises the stepof mixing the molten glass in a mixing tank prior to the forming step.The mixing tank is desirably lined, at least at a wall thereof incontact with the molten glass, with a lining material for glass meltingfurnaces, which material comprises platinum or platinum alloy as a basematerial containing osmium as an impurity in an amount no more than 20ppm, so that the wall evolves no bubbles during mixing.

[0022] Since each step of the process is carried out in the facilitieslined with the lining material containing osmium in an amount no morethan 20 ppm, bubble-free, high quality glass products can be obtained.More desirably, the osmium content in the lining material is reduced toless than 10 ppm so that glass products of higher quality can beobtained.

[0023] According to a fourth aspec of the present invention, there isprovided a process for purifying a lining material for glass meltingfurnaces, which comprises the steps of providing osmium-containingplatinum or platinum alloy, and keeping the osmium-containing platinumor platinum alloy at a temperature higher than a temperature at whichosmium oxide vaporizes, for a predetermined period of time, so that thecontent of osmium decreases to 20 ppm or less at least in a surface orits vicinity of the platinum or platinum alloy.

[0024] The osmium content exceeding 20 ppm can be reduced by keeping theplatinum or platinum alloy at 1700° C. for hundreds of hours.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Certain preferred embodiments of the present invention willhereinafter be described in detail, by way of example only, withreference to the accompanying drawing, in which:

[0026]FIG. 1 is a graph showing a relation between the content of osmiumin platinum and the number of bubbles.

[0027]FIG. 2 is a schematic diagram showing the general arrangement of aglass melting furnace and glass forming mold according to the presentinvention.

[0028]FIGS. 3A and 3B are schematic representations showing bubbles inglass samples.

[0029]FIG. 4 is a graph showing a relation between the glass temperatureand the glass viscosity in Experiment 1.

[0030]FIG. 5 is a graph showing a relation between the content of osmiumin platinum and the number of bubbles in Experiment 2.

[0031]FIG. 6 is a graph showing a relation between the glass temperatureand the glass viscosity in Experiment 2.

[0032]FIG. 7 is a graph showing changes in osmium contents with time.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] The following description is merely exemplary in nature and is inno way intended to limit the invention or its applications or uses.

[0034] In FIG. 1, the abscissa represents the content of osmium inplatinum heated to 1150° C. and 1300° C. while the ordinate representsthe number of bubbles. It is noted that when the osmium content falls ina range from 20 ppm to 31 ppm, the number of bubbles reaches a limitintolerable to product quality. This means that the formation of bubblescan be suppressed if the upper limit of osmium content is set at 20 ppmwith allowance, as indicated by dots P1 and P2.

[0035]FIG. 2 shows a glass melting furnace 1 according to the presentinvention. The glass melting furnace 1 consists of a melting tank 2, arefining tank 3, a homogenizing tank 4, mixing tank 5, and a formingunit 6, which are arranged in the order mentioned. They are internallylined with a platinum- or platinum-alloy-based material according to thepresent invention. The platinum-based lining material will be explainedlater. The lining of the melting tank may be omitted for the economy ofinstallation cost.

[0036] The term “platinum alloy” used herein means one which containsrhodium, or one which contains zirconium oxide and calcium oxide. Thelatter is so-called reinforced platinum of homogeneous dispersion type.

[0037] Glass raw materials are melted in the melting tank 2.

[0038] In the refining tank 3, the molten glass fed from the meltingtank 2 are freed of bubbles and dissolved gas therein by means of afining agent added to the glass raw materials.

[0039] The homogenizing tank 4 holds the fined molten glass therein,with or without cooling, so that it has a uniform temperaturethroughout.

[0040] In the mixing tank 5, the molten glass is stirred (at a uniformtemperature) so as to prevent cords. Designated by reference numeral 9is a platinum stirrer.

[0041] The forming unit 6 gets the molten glass made into a sheet glassby drawing.

[0042] These steps are carried out sequentially and continuously in aglass melting furnace 1.

[0043] Bubbles are possible throughout the steps and removed by therefining tank 3 (which is originally designed for refining) if they areformed in the melting tank 2 and the refining tank 3. However, theyusually remain unremoved if they are formed thereafter in thehomogenizing tank 4, mixing tank 5, forming unit 6, discharging unit 7,and glass forming mold 8. This is avoided in the present invention bylining with platinum or platinum alloy at least a rear facility, namely,homogenizing tank 4, mixing tank 5, forming unit 6, discharging unit 7,and glass forming mold 8.

[0044] For the refining tank 3 to carry out refining easily, the numberof bubbles formed in the melting tank 2 and refining tank 3 shouldpreferably be as small as possible. To this end, it is desirable to linethe melting tank 2 and refining tank 3 with platinum or platinum alloywhich hardly evolves bubbles.

[0045] In other words, the present invention is characterized in thatthe entire facility or the rear facility (homogenizing tank 4 andsucceeding installations) are lined with platinum or platinum alloywhich hardly evolves bubbles. In the latter case, the cost of equipmentcan be somewhat saved.

[0046] If requirements for quality associated with bubbles and cords arenot so stringent, the homogenizing tank 4 may be omitted or the meltingtank 2 and refining tank 3 may be combined together.

[0047] As shown in FIG. 2, the molten glass passed through the mixingtank 5 goes directly to the glass forming mold 8 where it is made intomolded glass, or indirectly to the glass forming mold 8 through thedischarging unit 7.

[0048] It is desirable that platinum or platinum alloy (specified in thepresent invention as the lining material for glass melting furnaces) beused also for the connectors 10 joining the melting tank 2, refiningtank 3, homogenizing tank 4, mixing tank 5, and forming unit 6 togethersequentially.

[0049] The glass raw materials are incorporated with fining agents suchas arsenic oxide, antimony oxide, tin oxide, barium chloride, bariumsulfate, and calcium chloride, which remain in the molten glass.

[0050] The above-mentioned glass melting furnace 1 can treat glass ofany composition, such as aluminoborosilicate glass, aluminosilicateglass, and alkali-free glass (containing substantially no alkali, likealkali-free aliminosilicate glass). They have a higher strain point thansoda lime silica glass for building window glazing.

[0051] The glass melting furnace 1 of the present invention can treatadequately electronic glass (such as a substrate for a liquid crystaldisplay) and substantially alkali-free aluminoborosilicate glass whichare required to be free of bubbles almost completely.

[0052] It is also suitable for melting and forming optical glass(including waveguides, optical lenses, and optical fibers) of theabove-mentioned composition, whose specification for bubbles per unitvolume is not so stringent as that for LCD substrates.

EXAMPLES

[0053] Five experiments as explained below were conducted to demonstratethat the platinum or platinum alloy specified in the present inventionhardly evolves bubbles.

[0054] (1) Laboratory experiment with test pieces of LCD glass.

[0055] (2) Laboratory experiment with test pieces of optical glass.

[0056] (3) Test on an actual glass melting furnace shown in FIG. 2.

[0057] (4) Test on an actual glass melting furnace shown in FIG. 2.

[0058] (5) Purification of platinum.

[0059] Experiment 1

[0060] Glass raw materials: as per composition shown in Table 2.

[0061] Platinum crucible: 200 ml in capacity, made of platinum bullionfree of osmium as impurities (not detectable by emission spectroscopicanalysis with a detection limit of ca. 1 ppm).

[0062] Melting temperature: 1600° C.

[0063] Melting time: 3 hours

[0064] Refining temperature: 1600-1650° C.

[0065] Glass sample: disc, about 70 mm in diameter and about 5 mm thick.cast onto a clean stainless steel board. TABLE 2 Glass composition mol %SiO₂ 67.4 B₂O₃ 11.0 Al₂O₃ 10.0 MgO  1.0 CaO  5.7 SrO  2.0 BaO  2.7 As₂O₃ 0.2 Total 100.0  Glass characteristics α_(100-300° C.) (10⁻⁷/deg) 37Strain point (° C.) 650  Viscosity at 1500° C. (poise)  <10³  

[0066] Platinum plates: Six samples were prepared from platinum bullioncontaining different amounts of osmium (1.0, 10.3, 20, 31, 976, 7460ppm), determined by emission spectroscopic analysis.

[0067] A glass sample cut to 1 cm square was placed on the platinumplate and heated at 1050° C., 1150° C., 1200° C., 1250° C., 1300° C., or1600° C. for 1 hour. It was examined for bubbles formed during heatingby means of a high-temperature microscope equipped with a videorecorder.

[0068]FIGS. 3A and 3B show, in top plan and side elevation, the glasssamples containing bubbles. FIG. 3A shows the glass sample which hadbeen heated at 1300° C. on a platinum plate containing 1 ppm osmium.FIG. 3B shows the glass sample which had been heated at 1300° C. on aplatinum plate containing 7460 ppm osmium.

[0069] In these figures, there are shown glass samples 11A and 11B,platinum plates 12A and 12B, and bubbles 13A and 13B. It is to be notedthat there is only one bubble in FIG. 3A while there are many bubbles inFIG. 3B.

[0070] Table 3 below shows the results of Experiment 1 in terms of thenumber of bubbles formed during heating per hour per unit area (cm²) ofthe platinum plate. TABLE 3 Heating Content of osmium in platinumViscosity of Temperature 1.0 ppm 10.3 ppm 20 ppm 31 ppm 976 ppm 7460 ppmglass (poise) 1050° C. 0 0 0 0.3 0.4 1.2 1.8 × 10⁶ 1150° C. 0.4 1.4 2.03.2 29 124 1.6 × 10⁵ 1200° C. 0.8 1.4 2.1 3.1 32 137 5.4 × 10⁴ 1250° C.0.5 1.5 2.3 2.9 34 146 2.2 × 10⁴ 1300° C. 0.7 1.6 2.3 4.1 31 168 9.6 ×10³ 1600° C. 0.7 1.5 2.5 5.0 40 229   3 × 10³

[0071] The data in Table 3 are partly graphed in FIG. 1. It was foundfrom FIG. 1 that the number of bubbles increases with the increasingcontent of osmium in the platinum plate. And, it turned out that bubblesare formed while glass is in contact with platinum at high temperaturesif platinum contains osmium as impurities.

[0072] Thus, it was confirmed experimentally that an effective way toreduce the number of bubbles is to reduce the content of osmium inplatinum as far as possible.

[0073] Table 3 above also shows the viscosity of glass (in poise) in therightmost column. It is noted that the higher the temperature, thegreater the number of bubbles and the lower the viscosity. This meansthat more bubbles are formed as the viscosity decreases. This in turnsuggests a certain relation between viscosity and bubble formation.

[0074] It is considered that bubbles move (rise) very slowly in moltenglass of high viscosity and hence new bubbles are hardly formed, whereasbubbles in molten glass of low viscosity easily move (rise), permittingnew bubbles to be formed one after another.

[0075] Data in the rightmost column of Table 3 are graphed in FIG. 4,with glass viscosity (on ordinate) plotted against temperature (onabscissa).

[0076] Table 3 indicates that no bubbles are formed at 1050° C. if theosmium content is no more than 20 ppm but bubble formation starts at1050° C. if the osmium content is more than 20 ppm.

[0077] This is signified by an arrow (marked “bubble formation”) at1050° C. in FIG. 4.

[0078] Similarly, an arrow marked “refining” appears at 1650° C. in FIG.4, because this temperature is the highest in the entire process. Recallthat the temperature is 1500-1650° C. in the refining tank, 1500° C. inthe homogenizing tank, 1500° C. in the mixing tank, and 1150-1200° C. inthe forming unit.

[0079] White arrow in FIG. 4 indicates the range of viscosity (2×10² to2×10⁶ poise) in which the formation of bubbles is likely. If platinumcontaining osmium no more than 20 ppm, preferably no more than 10 ppm,is used for the molten glass having a viscosity in this range, then itwould be possible to suppress the formation of bubbles. Conversely,using platinum with a low osmium content for the molten glass having aviscosity outside the above-mentioned range is not effective insuppressing bubbles but is uneconomical.

[0080] Experiment 2

[0081] Glass raw materials: as per composition shown in Table 4.

[0082] Platinum crucible: 200 ml in capacity, made of platinum bullionfree of osmium as impurities (not detectable by emission spectroscopicanalysis with a detection limit of ca. 1 ppm).

[0083] Melting temperature: 1350° C.

[0084] Melting time: 3 hours

[0085] Refining temperature: 1350-1400° C.

[0086] Glass sample: disc, about 70 mm in diameter and about 5 mm thick,cast onto a clean stainless steel board. TABLE 4 Glass composition mol %SiO₂ 72.6 B₂O₃ 11.0 BaO  0.6 Na₂O  7.8 K₂O  7.9 As₂O₃  0.1 Total 100.0 Glass characteristics α_(100-300° C.) (10⁻⁷/deg) 89 Strain point (° C.)546  Viscosity at 1500° C. (poise)  <10²  

[0087] Platinum plates: Six samples were prepared from platinum bullioncontaining different amounts of osmium (1.0, 10.3, 20, 31, 976, 7460ppm), determined by emission spectroscopic analysis.

[0088] A glass sample was placed on the platinum plate and heated at750° C., 800° C., 900° C., 1000° C., 1200° C., or 1400° C. for 1 hour.It was examined for bubbles formed during heating by means of ahigh-temperature microscope equipped with a video recorder. Observationswere the same as shown in FIGS. 3A and 3B.

[0089] Table 5 below shows the results of Experiment 2 in terms of thenumber of bubbles formed during heating per hour per unit area (cm²) ofthe platinum plate. TABLE 5 Heating Content of osmium in platinumViscosity of Temperature 1.0 ppm 10.3 ppm 20 ppm 31 ppm 976 ppm 7460 ppmglass (poise)  770° C. 0 0 0 0 0 0 1.5 × 10⁷  810° C. 0 0 0.1 0.2 0.51.3 2.2 × 10⁶  870° C. 0.2 1.1 1.6 2.8 30 127 1.3 × 10⁵  940° C. 0.4 1.32.0 3.2 33 131 1.6 × 10⁴ 1080° C. 0.4 1.7 2.3 3.7 35 163 1.0 × 10³ 1300°C. 0.5 1.8 3.0 4.8 41 221   2 × 10² 1400° C. 0.5 1.9 2.9 5.0 43 230   7× 10

[0090] The data in Experiment 2 (at heating temperatures of 870° C. and1080° C.) are graphed in FIG. 5, with the abscissa representing theosmium content and the ordinate representing the number of bubblesformed. It is noted that when the osmium content falls somewhere in therange from 20 ppm to 31 ppm, the number of bubbles reaches a limitintolerable to product quality. This suggests that the formation ofbubbles is suppressed if the upper limit of osmium content is set at 20ppm with allowance, as indicated by dots P3 and P4.

[0091] It was found from Experiment 2 (FIG. 5) that the upper limit ofthe osmium content should be 20 ppm, preferably 10 ppm, as in the caseof Experiment 1 (FIG. 1).

[0092] The data in the rightmost column of Table 5 are graphed in FIG.6, with glass viscosity (on logarithmic ordinate) plotted againsttemperature (on abscissa).

[0093] Table 5 indicates that no bubbles are formed at 810° C. if theosmium content is no more than 10.3 ppm but bubble formation starts at810° C. if the osmium content is more than 20 ppm.

[0094] This is signified by an arrow (marked “bubble formation”) at 810°C. in FIG. 6.

[0095] Similarly, an arrow marked “refining” appears at 1400° C. in FIG.6, because this temperature is the highest in the entire process. Recallthat the temperature is 1350-1400° C. in the refining tank, 1300° C. inthe homogenizing tank, 1200° C. in the mixing tank, and 1050° C. in theforming unit.

[0096] White arrow in FIG. 6 indicates the range of viscosity (7×10 to2×10⁶ poise) in which the formation of bubbles is likely. If platinumcontaining osmium no more than 20 ppm, preferably no more than 10 ppm,is used for the molten glass having a viscosity in this range, then itwould be possible to suppress the formation of bubbles. Conversely,using platinum with a low osmium content for the molten glass having aviscosity outside the above-mentioned range is not effective insuppressing bubbles but is uneconomical.

[0097] The way to suppress the formation of bubbles by controlling thetemperature of molten glass would vary depending on the glasscomposition, whereas the way to suppress the formation of bubbles bycontrolling the viscosity of molten glass would be the same regardlessof the glass composition.

[0098] It is known from FIG. 6 that in the case of optical glass havinga viscosity of 7×10 to 2×10⁶ poise the formation of bubbles iseffectively and economically suppressed if platinum containing less than20 ppm osmium, preferably less than 10 ppm osmium, is used as the liningmaterial for the glass melting furnace.

[0099] The formation of bubbles in optical glass (in FIG. 6) can besuppressed as a matter of course if the viscosity of the optical glassis in the range of 2×10² to 2×10⁶ poise as in the case of LCD glass (inFIG. 4) for which the formation of bubbles can be suppressed effectivelyand economically if its viscosity is in the same range as above.

[0100] Consequently, the formation of bubbles can be suppressed in anykind of glass so long as the viscosity is in the range of 2×10² to 2×10⁶poise and the osmium content is less than 20 ppm, preferably less than10 ppm.

[0101] Incidentally, the above-mentioned two kinds of glass exhibittheir lowest viscosity at temperatures corresponding to their refiningtemperatures.

[0102] To see if the results of the above-mentioned Experments 1 and 2(laboratory tests) are applicable to actual operation, Experiment 3 withactual equipment was conducted.

[0103] Experiment 3

[0104] Glass raw materials: as shown in Table 2 or the same one as inExperiment 1.

[0105] Equipment: as shown in FIG. 2. Platinum specified below is usedfor lining the refining tank, homogenizing tank, mixing tank, formingunit, and connectors (excluding the melting tank).

[0106] Platinum: containing as impurities 20 ppm osmium, 4 ppm arsenic,and 5 ppm iridium.

[0107] Melting tank: heated by natural gas and electric booster at about1600-1650° C. to melt continuously fed glass raw materials.

[0108] Refining tank: heated by natural gas and electric booster atabout 1600° C. to effect fining with the aid of As₂O₃ incorporated intoglass raw materials.

[0109] Homogenizing tank: electrically heated at about 1500° C.

[0110] Mixing tank: stirred by platinum stirrers at 1500° C.

[0111] Forming unit: designed to form sheet glass about 1000 mm wide and0.7 mm thick.

[0112] During operation from beginning, the resulting sheet glass wasexamined as to the number of bubbles therein per unit weight. It tookabout five days for the number of bubbles to decrease from 10 per kg to1 per kg.

[0113] After that, the number of bubbles decreased further to a constantvalue of 0-0.3 per kg. This value indicates that the sheet glass issatisfactory for use as an LCD substrate glass.

[0114] The results of Experiment 3 revealed that platinum containing 20ppm osmium contributes to the production of high-quality glass productswith very few bubbles.

[0115] It is concluded from the results of Experiments 1 and 3 that thecontent of osmium in platinum should preferably be no more than 10 ppmto reduce the number of bubbles.

[0116] Experiment 4

[0117] For comparison with Experiment 3, this experiment was conductedusing platinum of industrial grade containing 50 ppm osmium.

[0118] The glass melting and forming equipment used in this experimentis identical in structure and scale with the one used in Experiment 3.Continuous sheet glass (about 1000 mm wide) was produced and cut tosize. The composition of glass raw materials is shown in Table 2.

[0119] The refining tank and the forming unit were lined with platinumwhich contains as impurities 50 ppm osmium, 4 ppm arsenic, and 6 ppmiridium.

[0120] During operation from beginning, the resulting sheet glass wasexamined for the number of bubbles therein per unit weight. It tookabout one month for the number of bubbles to decrease from about 100 perkg to 1 per kg. After that, the number of bubbles did not decrease anymore.

[0121] This means that Experiment 4 took six times as long warming-upperiod as Experiment 3 (five days).

[0122] The short warming-up period as in Experiment 3 is economicallyadvantageous in view of the fact that the life of platinum (in contactwith molten glass at high temperatures in the refining tank and formingunit) is usually shorter than one year (depending on operatingconditions).

[0123] Experiments 3 and 4 are intended to verify Experiment 1 by actualoperation. It was confirmed that Experiment 2 is also applicable toactual operation although explanation is omitted here.

[0124] The foregoing demonstrates that the lining material for the glassmelting furnace should be platinum or platinum alloy containing osmiumno more than 20 ppm, preferably no more than 10 ppm.

[0125] Experiment 5

[0126] This experiment is intended for purification of platinum as alining material for glass melting furnaces.

[0127] Size of sample: 30×30×1.2 mm

[0128] Material of sample: platinum

[0129] Osmium content: 30 ppm

[0130] Apparatus for purification: electric furnace

[0131] Duration of purification: 100 hours and 360 hours

[0132] A sample of platinum containing 30 ppm osmium was heated in anelectric furnace at 1600° C. or 1700° C. and the content of osmium in itwas determined at certain intervals.

[0133] In the graph of FIG. 7, the content of osmium (ordinate) isplotted against time (abscissa).

[0134] It is noted that in the case of heating at 1600° C., it took 360hours for the osmium content to decrease to 20 ppm (desired value),whereas in the case of heating at 1700° C., the osmium content decreasedto 20 ppm within 260 hours and to 17 ppm within 360 hours.

[0135] In other words, in the latter case, decreasing the osmium contentby 10 ppm took 260 hours (11 days) by heating at 1700° C. and 360 hours(15 days) by heating at 1600° C.

[0136] Osmium in platinum is usually present in the form of metal. Uponheating in an oxidizing atmosphere, osmium in the surface layer ofplatinum changes into osmium oxide (OsO₄), which vaporizes and gets lostat 131° C. (boiling point). Then, osmium diffuses outward from insideand oxidizes in the surface layer.

[0137] The continuous and simultaneous diffusion and vaporization resultin osmium escaping from platinum. The fact that energy for diffusion ismuch greater than energy for vaporization is a conceivable reason thatthe higher the heating temperature, the shorter the period required fortreatment (purification).

[0138] Assuming that industrial platinum contains osmium in a maximalamount of 84 ppm (as shown in Table 1 above), it is necessary todecrease the osmium content by 64 ppm (from 84 ppm to 20 ppm). Timerequired to achieve this object would be simply calculated at 70 days(11 days multiplied by 6.4) in the case of heating at 1700° C. or 96days (15 days multiplied by 6.4) in the case of heating at 1600° C.

[0139] The same effect as above is obtained by heating at 1500° C. or1400° C. if the length of heating time is doubled. Therefore, heating at1600° C. or 1700° C. is desirable to save time for purification, asshown in Experiment 5.

[0140] An alternative to purifying purchased platinum or platinum alloyas mentioned above is to purchase platinum or platinum alloy which isalmost free of osmium, because the supplier of platinum bullion caneliminate osmium in the stage of refining.

[0141] The process of the present invention is suitable for theproduction of electronic glass to be free of bubbles almost completely.The absence of bubbles leads to high transparency and undistortedimages.

[0142] The process of the present invention is suitable particularly forthe production of LCD substrate glass. The absence of bubbles eliminatesthe possibility of circuit breakage.

[0143] The process of the present invention is also suitable for theproduction of alkali-free aluminoborosilicate glass. The absence ofbubbles leads to high transparency and undistorted images.

[0144] It is apparent that the formation of bubbles can be prevented ifthe platinum or platinum alloy specified in the present invention isused for lining that part of the glass melting furnace or glass formingunit which comes into contact with molten glass. Whether to line thefacility entirely partly is at one's discretion.

[0145] Obviously, various minor changes and modifications of the presentinvention are possible in the light of the above teaching. It istherefore to be understood that within the scope of the appended claimsthe present invention may be practiced otherwise than as specificallydescribed.

What is claimed is:
 1. A lining material for glass melting furnaces,comprising one of platinum and platinum alloy as a base materialcontaining osmium as an impurity in an amount no more than 20 ppm.
 2. Alining material for glass melting furnaces, as defined in claim 1,wherein the content of osmium is no more than 10 ppm.
 3. A glass meltingfurnace lined, at least at a part thereof in contact with molten glass,with a lining material for glass melting furnaces, which comprises oneof platinum and platinum alloy as a base material containing osmium asan impurity in an amount no more than 20 ppm.
 4. A glass melting furnaceas defined in claim 3, wherein the content of osmium is no more than 10ppm.
 5. A glass melting furnace as defined in claim 3, comprising amelting tank in which glass raw materials are melted, refining tank inwhich molten glass undergoes a refining treatment, and a forming unit inwhich molten glass is drawn and formed into sheet glass, at least saidforming unit being lined with said lining material.
 6. A glass meltingfurnace as defined in claim 3, comprising a melting tank in which glassraw materials are melted, a refining tank in which molten glassundergoes refining treatment, and a rear facility for homogenizing,mixing, and forming refined molten glass, said rear facility being linedwith said lining material.
 7. A glass melting furnace as defined inclaim 6, wherein said refining tank is lined with said lining material.8. A process for producing glass products, comprising the steps of:refining molten glass in a refining tank; homogenizing the molten glassfrom said refining tank at a predetermined uniform temperature in ahomogenizing tank; and forming the molten glass from said homogenizingtank into a sheet glass in a forming unit, said refining tank beinglined, at least at a wall thereof in contact with the molten glass, witha lining material for glass melting furnaces, which comprises one ofplatinum and platinum alloy as a base material containing osmium as animpurity in an amount no more than 20 ppm, so that said wall evolves nobubbles during refining, said homogenizing tank being lined, at least ata wall thereof in contact with the molten glass, with a lining materialfor glass melting furnaces, which comprises one of platinum and platinumalloy as a base material containing osmium as an impurity in an amountno more than 20 ppm, so that said wall evolves no bubbles duringhomogenization, said forming unit being lined, at least at a wallthereof in contact with the molten glass, with a material for glassmelting furnaces, which comprises one of platinum and platinum alloy asa base material containing osmium as an impurity in an amount no morethan 20 ppm, so that said wall evolves no bubbles during forming.
 9. Aprocess for producing glass products, as defined in claim 8, wherein thecontent of osmium is no more than 10 ppm.
 10. A process for producingglass products, as defined in claim 8, further comprising the step ofmelting glass in a melting tank prior to the refining step, said meltingtank being lined, at least at a wall thereof in contact with the moltenglass, with a lining material for glass melting furnaces, whichcomprises one of platinum and platinum alloy as a base materialcontaining osmium as an impurity in an amount no more than 20 ppm, sothat said wall evolves no bubbles during melting.
 11. A process forproducing glass products, as defined in claim 10, wherein the content ofosmium is no more than 10 ppm.
 12. A process for producing glassproducts, as defined in claim 8, further comprising the step of mixingthe molten glass in a mixing tank prior to the forming step, said mixingtank being lined, at least at a wall thereof in contact with the moltenglass, with a lining material for glass melting furnaces, whichcomprises one of platinum and platinum alloy as a base materialcontaining osmium as an impurity in an amount no more than 20 ppm, sothat said wall evolves no bubbles during mixing.
 13. A process forproducing glass products, as defined in claim 12, wherein the content ofosmium is no more than 10 ppm.
 14. A process for producing glassproducts, as defined in claim 8, wherein the glass products are thosefor electronic parts.
 15. A process for producing glass products, asdefined in claim 14, wherein the glass products for electronic parts areglass substrates for liquid crystal displays.
 16. A process forproducing glass products, as defined in claim 14, wherein the glassproducts for electronic parts are substantially alkali-freealuminoborosilicate glass.
 17. A process for producing glass products,as defined in claim 9, wherein the glass products are those forelectronic parts.
 18. A process for producing glass products, as definedin claim 17, wherein the glass products for electronic parts are glasssubstrates for liquid crystal displays.
 19. A process for producingglass products, as defined in claim 17, wherein the glass products forelectronic parts are substantially alkali-free aluminoborosilicateglass.
 20. A process for producing glass products, as defined in claim8, wherein the molten glass is prepared such that its viscosity does notexceed 2×10⁶ poise.
 21. A process for producing glass products, asdefined in claim 8, wherein the molten glass is prepared such that itsviscosity does not fall below 2×10² poise.
 22. A process for producingglass products, as defined in claim 20, wherein the molten glass isprepared such that its viscosity does not fall below 7×10 poise wherethe glass products are optical glass products.
 23. A process forproducing glass products, as defined in claim 21 or 22, wherein thecontent of osmium is no more than 10 ppm.
 24. A process for purifying alining material for glass melting furnaces, comprising the steps ofproviding one of osmium-containing platinum and platinum alloy, andkeeping said one of osmium-containing platinum and platinum alloy at atemperature higher than a temperature at which osmium oxide vaporizes,for a pre determined period of time, so that the content of osmiumdecreases to 20 ppm or less at least in a surface or its vicinity ofsaid one of platinum and platinum alloy.